In existing 2G wireless systems, according to applicable IS-95 standards, the messages transmitted on the reverse access channel (R-ACH) used by the mobile terminals within a base station's coverage area for providing a presence indicator to the network, for mobility management, and for indicating to the network when a call is to be placed, are at a fixed data rate of 4800 bps In newer 3G wireless systems (e.g., CDMA2000), supported by IS2000 standards, a variety of rates (e.g., 9600 bps, 19200 bps and 38400 bps) using the reverse enhanced access channel (R-EACH) can be supported. In accordance with the standards applicable to these newer systems, a base station advertises which rates it supports and a maximum allowed transmit time of a message in an overhead message that is transmitted to all the operating mobile terminals within the base station's coverage area. A mobile terminal, using the received information as to what rates are supported and the maximum transmit time for a message, and the size of the message it has to transmit, determines which rate is possible and then uses that one. If more than one rate is possible, then the mobile terminal, using whatever algorithm the mobile terminal manufacturer has incorporated into their mobile terminal, selects one rate from among the possible rates. With this scenario, no guidance is given to a mobile terminal by the base station in selecting which rate the mobile terminal should use to communicate with the base station, and the base station has no way of influencing the selection of the rate.
Selecting the highest data rate is advantageous to minimize channel latency since a message that takes 100 ms at 9600 bps may only take 25 ms at 38400 bps, a savings of 75 ms for each message on the channel. For applications where common channel latency is important, such as in real time applications like push-to-talk, this latency reduction is important. However, whereas it might be obvious to always select the highest supported rate to transmit at, in fact to achieve the same performance (i.e. Frame Error Rate [FER]), the mobile terminal will need to transmit at a higher power level. Since, the mobile terminals have a fixed maximum allowed output power level, the highest data rate may not always be the “best” data rate for transmission on the R-EACH. Further, a mobile terminal in selecting a rate might be more or less aggressive than the service provider would like. For example, if the message transmitted by the mobile terminal is a probe used for accessing the wireless system and mobile terminal selects a transmission rate that is too high, then one or more additional probes at increasing power levels will be transmitted by the mobile terminal as it attempts to get the base station to recognize and acknowledge the access request. As a result, this could have a deleterious effect on latency. In order for the mobile terminal to operate properly, therefore, it will either have to be too conservative in selecting a transmission rate, or if it selects too high a rate, will suffer the latency-degrading effect of possibly needed to transmit multiple probes until one is recognized. On the other hand, the latency benefits that could be achieved by transmitting at a higher rate are lost by operating conservatively and selecting a transmission rate that is lower than the rate at which the message could have been successfully transmitted to the base station.
A need exists, therefore, for dynamically determining the best transmission rate on the reverse common signaling channel in a wireless system.